Single and multiple deck conveyor dryers for reducing the moisture content of sheet materials, including green wood veneer, wherein the material being dried is conveyed through a stationary drying chamber while heated gases are circulated through the drying chamber, are well-known in the art. Evaporation of moisture from the material being dried causes an increase in the volume of gases in the dryer. Disposal of large amounts of gases containing volatile organic carbon (V.O.C.) which are evolved from the veneer during drying has been a major problem in the veneer drying art. Typically, gases are removed by an exhaust system and, in some systems, are exhausted directly to the atmosphere. By allowing gases to be exhausted directly to the atmosphere, pollutants have been allowed to escape and considerable quantities of heat energy are lost which is a considerable cost expense.
It is desirable to control the exhaust of gases from a wood veneer dryer to optimize the drying efficiency of the dryer and to provide a means for containing and treating the exhaust gases prior to discharge into the atmosphere. Such means include installing a V.O.C. separating device such as a catalytic or thermal oxidizer in the exhaust system. Such devices are well-known in the art. In order to optimize performance of these devices, it is preferable to maintain the temperature of the exhaust gases at or above a minimum operating temperature. At temperatures below this minimum operating temperature, pitch (i.e., condensed V.O.C. material) builds-up in the exhaust system, representing an obvious fire hazard. A fire in the exhaust system would result in costly repairs. While attempts have been made to optimize the performance of such exhaust systems, maintaining the temperature of the exhaust gases at or above the minimum operating temperature has proved difficult.
If the drying process is not carefully controlled and optimized, gases within a dryer of this type will be discharged through not only the exhaust system, but through input and output ends of the dryer because a positive pressure differential is created within the dryer relative to the external atmosphere. These prior art dryers have other disadvantages, including a loss of efficiency due to the entrance of air at ambient temperature into the dryer through input and output ends if a negative pressure differential is created within the dryer relative to the external atmosphere if gases are exhausted too quickly. Entrance of cooler ambient air reduces the temperature of the dryer, resulting in pitch-build up within the dryer and a real fire hazard.
There is accordingly a need in the art for a green wood veneer dryer wherein the temperature and pressure within the dryer are controlled to optimize performance of the dryer, prevent the build-up of pitch within the dryer and exhaust system, and prevent the outflow of gases containing V.O.C. from the dryer before treatment in the exhaust system.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.